A spectrometer is an optical system that conjugates an object in a superposition of chromatic images on the image plane in which a detector is located.
The images of each wavelength are translated in a direction, referred to as spectral direction, by an amount that depends upon the wavelength and follows a law of chromatic dispersion.
The object in the spectrometer is frequently an image coming from another optical system.
The object observed by the spectrometer is generally delimited by a rectangular diaphragm of field, referred to as slit.
The spatial direction and the spectral direction are defined with reference to the sides of the slit or of its images. The spatial direction is in general that of the major side of the rectangular of the slit, and the spectral direction is that of the minor side.
In all types of spectrometers, the image is formed by a superposition of chromatic images of the slit that are chromatically dispersed, i.e., translated in the spectral direction by an amount that depends upon the wavelength of the radiation.
The class of spectrometers is made up of generic spectrometers and imaging spectrometers.
There exists a substantial difference between a generic (non-imaging) spectrometer and an imaging spectrometer.
A non-imaging spectrometer performs a chromatic decomposition of the radiation coming from an extensive object (normally delimited by a rectangular-field diaphragm referred to as slit) and provides a measurement of the intensity of each chromatic component present in the object. This measurement is integrated in the spatial direction. This means that the detector situated on the focal plane of the spectrometer is unable to discriminate different points of the object (slit) in the spatial direction. In other words, if an electro-optical detector is used, it is generally a linear array.
Instead, in an imaging spectrometer, the detector is able to discriminate also in the spatial direction. In the case of electro-optical sensors, the array will be rectangular.
Accordingly, the quality of the chromatic images of the slit must be such as to enable resolution of details of the object in the spatial direction.
Basically the class of generic, i.e., non-imaging, spectrometers is a subclass of imaging spectrometers. The present invention can apply both to imaging spectrometers and to non-imaging spectrometers.
FIG. 1 is a generic representation of a scheme of a spectrometer in a so-called Gaertner configuration. The spectrometer is made of three basic parts: a collimator C, a chromatically dispersing system or dispersor D, and a focusing lens F. In the focus of the collimator C there is a slit S, which has a longitudinal development orthogonal to the plane of the figure.
An appropriate optical focusing system, not illustrated and extraneous to the spectrometer proper forms the image of the object to be analyzed on the slit S (if the object in question is at a distance a telescope will be used, whereas if the object is near an optical transport (relay) system, for example, a microscope lens, will be used).
The collimator C projects the image of the slit S at infinity, transforming the diverging beam f1 of rays coming from any point of the slit into a beam f2 of parallel rays. The inclination of this beam varies with the object point from which it comes in the direction normal to the drawing.
The rays thus collimated traverse the dispersing system D and are deviated, with different angles, according to the wavelength. Finally, the focusing objective F focuses the rays that have the same direction into one and the same image point. Consequently, images of the slit having different colors are formed on the focal plane P, said images varying their position in a direction orthogonal to the length of the slit.
The Gaertner configuration enables spectrometers to be made having focal distances of the collimator C and of the focusing objective F that are not necessarily equal. Consequently, magnifications other than 1× can be obtained.
An example of spectrometer of this type is described in EP-A-0316802. The dispersor generically designated by D in FIG. 1 may be made up of one or more components, in the form of prisms (refractive dispersor), diffraction gratings (diffractive disperser), or mixtures of both (prisms and gratings, the so-called “grisms”).
Using refractive or prismatic dispersors in an imaging spectrometer or diffraction gratings provided on curved surfaces, there may arise a phenomenon, which is generally undesirable, referred to as “curvature of the image of the slit”, or “curvature of slit”, or “smile”. This phenomenon is illustrated in FIG. 2, where a number of ideal image points from PO to P8 are represented, which are marked by a black dot and which are located on the perimeter of a rectangular grid, which has a height in the so-called “spatial direction” (vertical in FIG. 2) equal to the length of the slit, and a length (in the horizontal direction) corresponding to the extent, in the direction of chromatic dispersion, of the dispersed chromatic band. These points are as follows:
At the center of the slit:    P4 at one extreme of the dispersed chromatic band    P5 at the other extreme of the band    P0 at the center of the chromatic band
At the top end of the slit:    P1 at one extreme of the dispersed chromatic band    P2 at the center of the chromatic band    P3 at the other extreme of the band
At the bottom end of the slit,    P6 at one extreme of the dispersed chromatic band    P7 at the center of the chromatic band    P8 at the other extreme of the band.
The “true” images of the slit for three different colors are indicated by thick lines. The points from P′1 to P′8 represent the real images, affected by the distortions of the spectrometer, of the corresponding points from P1 to P8.
The curvature of the image of the slit or “smile” is the horizontal distance (i.e., along the spectral direction) of the real image points from the corresponding ideal image points. The smile is a function of the height h of the point considered on the slit and of the wavelength λ.
In addition to the above error, in this kind of apparatus there may also occur a so-called spatial co-registration error. The co-registration error is the distance of a “real” image point from its homologous ideal image point, measured in the spatial direction instead of in the spectral direction. This is indicated by SCRE in FIG. 5. This type of error derives from a chromatic variation of the magnification as a function of the field of view.
In addition to the errors referred to above, i.e., smile and spatial coregistration, in making a spectrometer it is necessary to take into account axial and extra-axial geometrical and chromatic aberrations, including curvature of field, which occurs when the image, instead of lying on a plane, lies on a curved surface (to a first approximation on a spherical cap). Since in an imaging spectrometer the sensitive elements of the detector generally lie in a plane, this aberration is highly undesirable and must be contained within the depth of focus or of field of the optical system, which is linearly dependent upon the wavelength and quadratically dependent upon the speed or f number. The variation in the size of the image of a point, due to curvature of field, is quadratically dependent upon the distance from the center, i.e., upon the height of the field of view.
On the other hand, even more important is the correction of aberration, and in particular of curvature of field, for systems with small f numbers (speed or f number is given by the ratio A=focal length/effective maximum diameter), i.e., ones with larger apertures. The possibility of working with low f numbers constitutes an important prerogative for a high-performance imaging spectrometer. A larger extension of the field of view is another very important feature 4 or an imaging spectrometer.
Correction of curvature of field, together with correction of other forms of aberration, enables a better resolution of the optical system to be achieved and hence enables use of detectors with pixels of smaller dimensions. This leads to systems with shorter focal lengths and consequently to systems of smaller dimensions. Of course, given the same resolution and the same radiometric efficiency, the smaller the pixel, the greater must be the aperture of the spectrometer, and hence the smaller the f number.